Meaning one: what China calls black tea
Chinese calls the category 红茶 — hong cha, literally red tea — after the colour of the liquor. English calls the same category black, after the colour of the dry leaf. They are the same teas: Keemun, Dianhong, Lapsang Souchong, and by extension Assam and Ceylon. So a Chinese or Japanese speaker saying "red tea" means what an English speaker calls black tea, and this is not a translation error on either side — the two languages named the category after different things. The complication is that Chinese ALSO has a category English does not translate at all: 黑茶, hei cha, literally black tea, which is the post-fermented dark category including pu-erh and the compressed bricks. So Chinese "black tea" and English "black tea" are different things, and Chinese "red tea" is English "black tea". This trips up experienced people regularly.
Meaning two: rooibos
Rooibos is Afrikaans for "red bush", and it is marketed in English as red tea or redbush tea. It is Aspalathus linearis, a South African legume, and it is not tea in any sense — different family, different continent, no caffeine. The name works commercially precisely because it sits beside tea on the shelf and the liquor is red. It is also the reason a reader who has learned that "red tea" means Chinese black tea will be wrong about half the products carrying the phrase.
Meaning three: hibiscus
Hibiscus infusion is deep red and is sold as red tea in several markets, particularly where it is a national drink under another name — karkadé in Egypt and Sudan, bissap in the Sahel, agua de Jamaica in Mexico, sorrel in the Caribbean. It is also the colouring and souring agent in a very large share of the fruit infusions sold in Europe, so a red "berry tea" is frequently a hibiscus product whatever the fruit on the box.
Why the collision matters practically
CAFFEINE. Chinese red tea contains caffeine. Rooibos and hibiscus contain none. Somebody avoiding caffeine who buys "red tea" on the strength of one meaning may get the other, and that is the failure mode with actual consequences. BREWING. The three want completely different handling. Chinese black tea steeps briefly and turns astringent if pushed. Rooibos tolerates boiling water and a long steep without turning bitter. Hibiscus is sharply acidic and is drunk cold and sweet in every tradition that drinks it seriously. SHELVING. All three sit in the tea aisle and only one is tea.
How to tell which you have in one glance
Look at the dry material. Chinese black tea is dark twisted leaf. Rooibos is fine red-brown needles with a slightly woody look. Hibiscus is dark red-purple, papery and irregular — the dried calyces of a flower, unmistakable once seen. Or read the ingredient list, which will say Camellia sinensis, Aspalathus linearis or Hibiscus sabdariffa, and which is the one part of the packet that cannot equivocate.